Current velocity as a factor in determining macroinvertebrate assemblages on wood surfaces

Abstract

The velocity of moving water near submerged wood surfaces may be an important factor for shaping benthic communities in sandy-bottomed streams. As wood can be suspended above the streambed and be kept free of inundating sediments, wood substrates may provide the primary hard surface for macroinvertebrate colonization. To examine this, we sampled macroinvertebrate assemblages from the surface of 20 pieces of submerged wood in a sandy-bottomed stream in the west-central Wisconsin. Near-surface velocity was characterized at the wood surface and the streambed macroinvertebrate assemblage was sampled near each wood piece for comparison. We found that the density of macroinvertebrates was five times greater on wood than the sandy streambed and the average richness on wood was twice as great. Macroinvertebrate abundance and richness on wood surfaces increased with velocity; however, abundance increased as a consequence of adding taxa rather than adding individuals of the dominant taxa. These data suggest that near-surface current velocity on wood may be an important determinant of benthic community composition when the availability of other hard surfaces is limited.

Keywords:

• current velocity
• wood
• macroinvertebrates
• species richness
• streams

Two factors known to influence macroinvertebrate abundance and distribution in streams are submerged wood (Coe et al. 2009; Entrekin et al. 2009) and current velocity (Hart and Finelli 1999; Fenoglio et al. 2004). Many benthic organisms are associated with coarse woody habitat (Hoffmann and Hering 2000) or are known to show preferences for certain ranges of velocity (Wellnitz et al. 2001; Franken et al. 2006). Wood provides hard and persistent surfaces for colonization by macroinvertebrates and may be less subject to burial by flow-driven sediments than mineral substrates. Wood substrates may be of particular importance in sandy-bottomed streams because fallen trees and snags may constitute virtual oases of stable habitat. Trees that fall into streams are often fixed to the bank, allowing portions of the submerged trunk to be suspended in the current and free of shifting sediments. Similarly, branches protruding vertically into the water from over-hanging trunks are exposed to a wide range of current velocities. These unique features of woody substrate suggest that the movement of water across wood surfaces may have a special role in structuring benthic communities in sandy-bottomed streams. To date, the influence of near-wood surface velocity on the structure of wood-inhabiting macroinvertebrate communities has gone largely unstudied.

Previous research has examined the coarse-scale effects of flow (Angradi et al. 2009) or the effects of flow disturbance on wood-associated communities (Hax and Golladay 1998; Spanhoff et al. 2006), but fine-scale, near-wood surface velocity effects have not been assessed. To explore this, we examined macroinvertebrate assemblages found on isolated pieces of wood across a range of near-surface velocities in a sandy-bottomed Wisconsin stream. Our objective was to determine how current velocity near wood surfaces related to the abundance and richness of the macroinvertebrate communities was found there.

Beaver Creek is a third-order, sandy-bottomed stream in the west-central Wisconsin (N 44° 48′ 48.50″, W 91° 16′ 11.77″) in which wood substrata and stream-spanning logjams are interspersed. On 20 July 2011, we studied a 250 m reach and sampled 20 submerged logs (mean ± SE; diameter = 15.3 ± 2.0 cm, length = 6.7 ± 0.70 m) with a circular, rubber plot (15 cm diameter, area = 177 cm²) placed on the upper surface of each, which provided a practical location for the ease of sampling and current velocity measurement. Macroinvertebrates were removed from the plot area using a toothbrush and caught in a hand net held immediately downstream. Three surface velocity measurements (0–1 cm resolution) were taken 5 cm apart at the surface of the wood and perpendicular to flow within each quadrat using a MiniWater20 Micro current velocity probe (Schiltknecht Messtechnik AG, Zürich, Switzerland). As the majority of macroinvertebrate diversity occurred on wood surfaces, velocity measurements were not taken at the streambed. To compare macroinvertebrate assemblages on wood with the surrounding streambed, we used a ‘mini’ Surber sampler (240 cm² sample area) to sample the streambed 1 m downstream of each log. At each wood piece the depth to sample area, diameter at sample area, and submerged length of the wood piece were also measured. To assess surface roughness, the decay class of each wood piece was ranked into four classes: class 1 wood pieces had all bark and small twigs (<3 cm) intact, class 2 had all bark intact but twigs were missing, class 3 had trace amounts of bark and a hard wood surface, and class 4 had lost all bark and had a soft wood surface. This was a modification of the Lienkaemper and Swanson (1987) method. Collected macroinvertebrates were identified to genus (except Diptera were identified to family level as higher resolution identification is time consuming) and the data were standardized by area and analyzed using linear regression and paired t-tests. Abundance data were log-transformed to improve the homogeneity of variance. Statistical tests were performed using JMP statistical software (v8.0, SAS Institute, Cary North Carolina, USA).

Sampled wood surfaces showed a positive relationship between the near-surface velocity and macroinvertebrate abundance (F _1,18 = 11.64, p = 0.003, r ² = 0.39); however, no single taxon drove this relationship. In fact, the only taxon-specific relationship with velocity that occurred was a negative one for the mayfly Heterocleon (F _1,5 = 28.04, p = 0.003, r ² = −0.85), while the others showed no relationship at all (p > 0.05). Additionally, there were no significant differences (p > 0.05) in abundance between slow (0–0.20 cm s⁻¹), medium (0.21–0.35 cm s⁻¹), and fast (>0.35 cm s⁻¹) velocity ranges for major taxa. This meant that abundance increased with velocity as a consequence of adding new taxa rather than adding individuals of the dominant taxa (Figure 1a). Corroborating this pattern, taxon richness had a strong, positive relationship to surface velocity (F _1,18 = 13.49, p = 0.001, r ² = 0.43; Figure 1b). We found no relationship between taxonomic richness or abundance and other physical factors of the wood (depth, surface roughness, length, or diameter).

Figure 1. The effect of current velocity on the structure of macroinvertebrate communities inhabiting wood surfaces in Beaver Creek. Macroinvertebrates were collected from 20 submerged logs using a 177 cm² quadrat and the log number of individuals (a) and taxonomic richness (b) was determined for each sample.

The densities of macroinvertebrates in Beaver Creek were higher on wood substrates. Out of the 468 individuals representing 19 taxa that were collected, five times more individuals were found on wood than on the streambed (mean ± SE; 18.5 ± 4.0 versus 4.9 ± 0.9 individuals cm⁻²; t = 3.8, p < 0.001) and the average richness on wood was twice as great (four versus two species; t = 3.90, p < 0.001). All taxa found on sand substrates were also present on wood substrates. Most common taxa were the Chironomidae (30% of individuals), followed by the Simuliidae (17%), the caddisfly Brachycentrus (16%), and the mayfly Baetis (14%).

Current velocity played a significant role in shaping benthic communities on exposed wood surfaces in Beaver Creek. Wood surfaces supported macroinvertebrate assemblages having greater abundance and taxonomic richness than the surrounding streambed. Each of these variables showed a positive response to near-surface velocity, but the greater number of taxa found in faster velocity drove both trends. There are at least two mechanisms that may explain this pattern. Increased current velocity can provide benthic organisms with resource subsidies by increasing delivery rates of particulate food for filter-feeders (Lancaster and Downes 2010) or effectively concentrating nutrients for benthic algae (Biggs et al. 1998), which serve as food for macroinvertebrate grazers. Enhanced resources can not only increase taxa numbers and richness but may also expand niche space (Sircom and Walde 2011). Niche expansion is a second process that may explain the assemblage patterns we found on wood. As near-surface velocity increases, so does the variance such that areas exposed to fast current often contain high heterogeneity in velocity (Wellnitz et al. 2001), thus expanding another important niche axis (Hart and Finelli 1999). Regardless of the mechanism, current velocity has the potential to be a key factor shaping benthic assemblages on wood, a critical habitat for macroinvertebrate communities in sandy-bottomed streams.

Acknowledgments

Authors thank the Office of Research and Sponsored Programs at the University of Wisconsin – Eau Claire, and a National Science Foundation CAREER grant to Todd Wellnitz (DEB-0642512) for funding this work. They also thank the Beaver Creek Reserve for allowing access to their land, and to Zachary Snobl, Aaron DeVoe, and Stephanie Vinetas for providing field assistance.